Tag Archives: cooking

Fresh fruit and vegetables are great, but where’s the best place to store them?

I’ve mentioned before that my Dad is a professional plant-wrangler (if you’ve never read the electric daisies post, do go and have a look – it’s a little-read favourite) and he often brings me home-grown fruit and vegetables.

What follows is an inevitable disagreement about storage, specifically, my habit of putting everything in the fridge.

In my defence, modern houses rarely have pantries (boo) and we don’t even have a garage. We do have a shed, but it’s at the bottom of our poorly-lit, somewhat muddy garden. Do I want to traipse out there on a cold, dark, autumn evening? No, I do not. So the fabled “cool, dark place” is a bit of problem. My fridge is cool and dark, I have argued, but here’s the thing – turns out, it’s too cool. And quite probably too dark.

This I have learned from the botanist James Wong (@botanygeek on Twitter), whose talk I attended on Monday this week at the Mathematical Institute in Oxford. James, it turns out, had a rather similar argument with his Mum, particularly regarding tomatoes.

We should’ve listened to out parents, because they were right. A lot of fruit and vegetables really are better stored outside of the fridge, and for tomatoes in particular “better” actually means “more nutritious”.

Lycopene is a very long molecule with lots of double C=C bonds.

Tomatoes, James explained, contain a lot of a chemical called lycopene. It’s a carotene pigment, and it’s what gives tomatoes their red colour.

Lycopene has lots of double bonds between its carbon atoms which form something chemists call a conjugated system. This has some rather cool properties, one of which is an ability to absorb certain wavelengths of light. Lycopene is especially good at absorbing blue and green wavelengths, leaving our eyes to detect the red light that’s left.

Tomatoes and lycopene also seem to have a lot of health benefits. There’s some evidence that lycopene might reduce the risk of prostate and other cancers. It also appears to reduce the risk of stroke, and eating tomato concentrate might even help to protect your skin from sun damage (don’t get any ideas, you still need sunblock). Admittedly the evidence is currently a bit shaky – it’s a case of “more research is needed” – but even if it turns out to that the causative relationship isn’t terribly strong, tomatoes are still a really good source of fibre and vitamins A, C and E. Plus, you know, they taste yummy!

But back to the fridge. Surely they will keep longer in the fridge, and the low temperatures will help to preserve the nutrients? Isn’t that how it works?

Well, no. As James explained, once tomatoes are severed from the plant they have exactly one purpose: to get eaten. The reason, from the plant’s point of view, is that the critter which eats them will hopefully wander off and – ahem – eliminate the tomato seeds at a later time, somewhere away from the parent plant. This spreads the seeds far and wide, allowing little baby tomato plants to grow in a nice, open space with lots of water and sun.

For this reason once the tomato fruit falls, or is cut, from the tomato plant it doesn’t just sit there doing nothing. No, it carries on producing lycopene. Or rather, it does if the temperature is above about 10 oC. Below that temperature (as in a fridge), everything more or less stops. But, leave a tomato at room temperature and lycopene levels increase significantly. Plus, the tomato pumps out extra volatile compounds – both as an insect repellant and to attract animals which might usefully eat it – which means… yes: room temperature tomatoes really do smell better. As if that weren’t enough, chilling tomatoes can damage cell membranes, which can actually cause them to spoil more quickly.

In summary, not only will tomatoes last longer out of the fridge, they will actually contain more healthy lycopene!

Anecdotally, once I got over my scepticism and actually started leaving my tomatoes on my windowsill (after years of refrigeration) I discovered that it’s true. My windowsill tomatoes really do seem to last longer than they used to in the fridge, and they almost never go mouldy. Of course, it’s possible that I might not be comparing like for like (who knows what variety of tomato I bought last year compared to this week), but I urge you to try it for yourself.

James mentioned lots of other interesting bits and pieces in his talk. Did you know that sun-dried shiitake mushrooms are much higher in vitamin D? Or that you can double the amount of flavonoid you absorb from your blueberries by cooking them? (Take that, raw food people!) Storing apples on your windowsill is likely to increase the amount of healthy polyphenols in their skin, red peppers are better for you than green ones, adding mustard to cooked broccoli makes it more nutritious, and it would be much better if we bought our butternut squash in the autumn and saved it for Christmas – it becomes sweeter and more flavoursome over time.

In short, fascinating. Who wants to listen to some “clean eater” making it up as they go along when you can listen to a fully-qualified botanist who really knows what he’s talking about? Do check out the book, How to Eat Better, by James Wong – it’s packed full of brilliant tidbits like this and has loads of recipes.

Christmas preparations are well underway by now, but have you been paying attention to your chemistry? Of course you have! Well, let’s see… (answers at the bottom, this is a low-tech quiz).

Let’s start with an easy one. In the nativity, the three wise men allegedly turned up at the stable with three pressies for little Jesus. But which chemical symbol could represent one of the gifts?
a) Ag
b) Au
c) Al

On the topic of chemical symbols, which christmassy word can you make out of these elements?
carbon, radium, carbon (again), potassium, erbium, sulfur

It doesn’t look like snow is very likely in most of England this year, but we can dream. And while we’re dreaming: why do snowflakes always have six sides?
a) because water has three atoms and they join up to make six.
b) it’s usually something do with hydrogen bonding.
c) they don’t, it’s a myth.

Where would you be most likely to find this molecule at Christmas?
a) In the Christmas cookies.
b) In the festive stilton.
c) In the Christmas turkey.

Mmm Christmas cookies! But which other chemical substance is often added to cakes and biscuits to help them rise?
a) sodium carbonate.
b) sodium hydrogen carbonate.
b) calcium carbonate.

Let’s think about the booze for a moment. Which fact is true about red wine?
a) It tastes significantly different to white wine.
b) Mixing it with other drinks will make your hangover worse.
c) It’s mostly water.

And why are beer bottles usually brown or green?
a) Because these colours block blue light.
b) Because in the old days beer was often a funny colour, and the coloured glass disguised it.
c) Because it’s good luck.

Where would you be most likely to find this molecule at Christmas?
a) In the Christmas cake
b) In the mulled wine
c) In the wrapping paper

Let’s turn to New Year for a moment. What makes party poppers go pop?
a) Gunpowder
b) Silver fulminate
c) Armstrong’s mixture

And who doesn’t love a firework or two? So, which substance is used to produce a blue colour?
a) Sodium bicarbonate
b) Copper chloride
c) Magnesium powder

ANSWERS

b) Au – gold

CRaCKErS!

b) – hydrogen bonds form between the oxygen atom of one water molecule and the hydrogen atom of another molecule, causing the molecules to link up into hexagon shapes (pretty much any question to do with water can be answered with ‘something to do with hydrogen bonding’).

a) – in the cookies, it’s cinnamaldehyde, which is the molecule that gives cinnamon it’s flavour and smell.

b) – sodium hydrogen carbonate, also known as sodium bicarbonate, or just ‘bicarb’, breaks down when heated and forms carbon dioxide. It’s the formation of this gas which causes mixtures to rise.

c) – the flavour and colour components of wine only make up about 2% of its volume. If we assume 12% alcohol, then the wine is 86% water. Still, probably best not to glug on a wine bottle after your morning run. On the other two points, there isn’t much evidence that mixing drinks makes hangovers worse (unless, as a result, you drink more alcohol), although some specific types of drinks may cause worse symptoms than others. As for taste, in this paper researchers describe an experiment where they gave 54 tasters white wine dyed red with food colouring. The tasters then went on to describe it as a red wine, suggesting that appearance was much more important than actual taste.

a) – the coloured glass used in beer bottles is there to block blue light. These wavelengths can cause some of the substances in beer to react with each other, resulting in unpleasant flavours.

c) – in the wrapping paper. It’s cellulose, the main constituent of paper.

c) – It’s usually Armstrong’s mixture in party poppers, which is a highly sensitive primary explosive containing red phosphorous (eek). Did I trick any of the chemists out there? Silver fulminate is used in Christmas crackers.

b) – Copper chloride, and also copper oxide and copper carbonate when combined with other things. Sodium bicarbonate produces yellow, and magnesium is white.

It’s come up before of course, but there’s a lot of chemistry in cooking. I do like tinkering with recipes: all that lovely weighing things on digital scales, measuring liquids, working out ratios and tweaking the exact sequence of steps – what more could a chemist want? I spent ages working on my chocolate brownie recipe when I should have been writing up my PhD thesis (it does produce excellent chocolate brownies, so I maintain it was a entirely valid use of my time).

Last week fate transpired to drop more than one reference to ‘biscuits’ in my lap. Now, these were American sources, so I was aware that they weren’t talking about what we call biscuits (and Americans call cookies), not least because in one of them there was talk of making a ‘biscuit sandwich’ that included sausage. Now, I like a chocolate digestive as much as the next person, but I wouldn’t slap a chunk of grilled pork product in between two of them and call it breakfast.

So I decided to try and find a recipe. And, after a bit of faffing around converting Fahrenheit to Celsius and cups to grams (honestly, I do understand the principle of cups and baking by ratio, but is it really easier to measure out a cup of butter than just use scales?) I finally came up with a workable recipe.

Turns out American biscuits are basically sugar-less scones. Who knew.

What’s the chemistry connection? Well, just like scones, the raising agent in American biscuits is baking soda, or sodium hydrogen carbonate. It causes the mixture to rise because it does this when it’s heated:

2NaHCO3 –> CO2 + H2O + Na2CO3

This type of reaction is called thermal decomposition, because the heat is causing the sodium hydrogen carbonate (NaHCO3) to break apart. The carbon dioxide (CO2) is a gas and produces lots of lovely bubbles that make your finished product nice and light. Water (H2O) is also a product, which helps to keep everything nice and moist.

This clever bit of cookery chemistry starts to happen slowly at 50 oC, but once you get over 200 oC (a more typical baking temperature) it’s pretty fast. So much so that you can bake your biscuits for just 12 minutes or so and they’ll be perfectly risen. Contrary to common belief, there’s no need to add some kind of acid to the mixture (buttermilk is often mentioned). Acids do react with carbonates to produce carbon dioxide, but there’s no need – heat will do the job for you.

So without further ado, here’s my tinkered recipe. It’s really great this, it literally only takes 10 minutes plus baking time, and you probably have all these ingredients already:

Ingredients

360 g plain white flour

4 tsp of baking soda (sodium hydrogen carbonate)

1 tsp sugar

½ tsp salt

75 g cold (straight from the fridge) unsalted butter, cut into cubes

230 g* milk

(*If you prefer to use a jug this is as close to 230 ml as makes no difference, since milk is mostly water and water has a density of 1 g/ml, but it saves washing up to just stick the bowl on the scales and weigh it.)

Method

Heat the oven to 230 oC. It needs to be nice and hot, so turn it on in good time.

Measure the dry ingredients in a large bowl and mix them.

Using clean, cold hands rub the butter into the dry ingredients until the mixture resembles fine breadcrumbs and there are no lumps of butter.

Pour in the milk and mix with the flat of a knife until the dough comes together.

Take the dough out of the bowl and place it on a lightly floured surface. Knead it gently a few times until it forms an even ball and has an elastic, ever so slightly sticky, texture.

Press into a rough oblong, about 2 cm thick. Cut the dough into six roughly equal pieces (you can use cookie cutters, but again, why create unnecessary washing up). Place these on a greased baking tray.

Bake for about 12 minutes, until the biscuits are golden brown (the colour is, of course, courtesy of another bit of chemistry: the Maillard reaction).

Transfer them to a rack to cool, but no need to leave them too long – they’re best eaten warm!

Split the biscuits in half and fill them with anything you like, savoury or sweet. They’re delicious served plain with lashings of butter. As a more substantial lunchtime snack, try cheese and ham. Lemon curd has also proved a favourite. If you have leftovers they will keep until the next day if wrapped up, and are especially nice toasted and buttered.

And there you are, a metric version of the classic American biscuit recipe, with a bit of chemistry thrown in. I think this might be a first. Heston eat your heart out.